2. Electrical - Ijeeer - A Novel Hybrid Asymmetric - Vinod Kumar k

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International Journal of Electrical and

Electronics Engineering Research (IJEEER)

ISSN(P): 2250-155X; ISSN(E): 2278-943X

Vol. 4, Issue 3, Jun 2014, 11-22

© TJPRC Pvt. Ltd.

A NOVEL HYBRID ASYMMETRIC CASCADED MULTILEVEL PWM INVERTER FOR

REDUCTION OF THD

VINOD KUMAR K1, M. DILEEP KRISHNA

2 & BADUGU PRAVEENA

3

1,2Assistant Professor, Department of EEE, GITAM University, Visakhapatnam, Andhra Pradesh, India

3Assistant Professor, Department of EEE, GVP College of Engineering for Women, Visakhapatnam,

Andhra Pradesh, India

ABSTRACT

When asymmetrical inverter is operated in PWM mode it gives high value of total harmonic distortion (THD)

which is mainly due to unequal pulses in the each stage of multilevel output. To avoid this hybrid asymmetrical inverter is

proposed in this paper with asymmetrical and symmetrical parts. The main objective is to get the same THD value in

asymmetrical inverter as same level symmetrical inverter when both are operated in PWM mode. This proposed inverter

uses few levels at high frequency and remaining at low frequency. Henceforth this proposed PWM operation which uses

more number of low frequency switches gives the better output as that obtained in asymmetric inverter. This decreases the

cost of the inverter and increases the reliability of the inverter. Further with this inverter dv/dt across the high frequency

switches can be decreased and all odd levels which are not possible with the asymmetric inverter can be obtained.

KEYWORDS: Cascaded Multilevel Inverter, Hybrid Asymmetrical Inverter, Pulse Width Modulation, Symmetrical

Inverter, Total Harmonic Distortion

INTRODUCTION

The cascaded H-bridge multilevel inverters can be divided into two groups from the viewpoint of values of the

dc voltage sources: the symmetric and the asymmetric topology. In the symmetric topology, the values of all of the dc

voltage sources are equal. This characteristic gives the topology good modularity. However, the number of the switching

devices rapidly increases by increasing the number of output voltage level. In order to increase the number of output

voltage level, the values of the dc voltage sources are selected to be different, these topologies are called asymmetric [1].

The advantage of an asymmetric cascaded inverter is an increased number of voltage levels for a given modules count.

If the dc voltages of individual cells are arranged according to a geo metric progression with a common ratio of 2 [2], then

the output voltage levels count grows with a modules number approximately as a power of 2. For converter cell dc voltages

arranged as a geometric progression with a common ratio of 3, the levels count grows as a power of 3 of cells number.

In spite of the above-mentioned advantages, this type of inverters has got some disadvantages. One is the unequal power

generated by the existing dc sources. As a result, the life time of the sources applied in bridges would be different.

Therefore, the number of checkups, substitution and battery changing time increases and as a result the

maintenance cost of inverter increases too. In many industry applications it is often necessary to control the output voltage

of asymmetrical inverters in order to cope with the variations of the dc input voltage, to regulate the voltage of inverters,

and to satisfy the constant volts and frequency control requirement. Most efficient and heuristic method of controlling the

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12 Vinod Kumar K, M. Dileep Krishna & Badugu Praveena

Impact Factor (JCC): 5.9638 Index Copernicus Value (ICV): 3.0

output voltage is to incorporate pulse width modulation within the inverter [3] & [4]. By using PWM techniques the

fundamental voltage of inverter as well as harmonics can be controlled.

PROPOSED CASCADED MULTILEVEL INVERTER

A new multilevel inverter topology is proposed by using series-connected sub multilevelconverters.

Figure 1 shows the basic unit for a submultilevel converter [5] & [6].

Figure 1: Basic Unit for a Sub-Multilevel Converter

The proposed topology of inverter uses submultilevel inverter part and polarity creator part as shown in Figure 2.

The submultilevel converter gives either zero or positive output as shown in Figure 3.

When the proposed inverter operates as a symmetrical inverter, Vi=Vdc where as in asymmetrical inverter Vi is

equal to either Vi= 2(i−1)

Vdc or 3iVdc wherei=1, 2,···, n [7].

CONTROL STRATEGY

Control techniques used for the proposed symmetrical or asymmetrical inverter are based on fundamental

frequency modulation or pulse width modulation techniques. Out of these techniques fundamental frequency operation is

easy to control.

To apply conventional PWM techniques for the proposed topology it needs some modifications. Proposed PWM

technique is derived from level shifted carrier PWM technique.

VnS2n-1

S2n

VO

V2

V1

S3

S1

S4

S2VO,1

VO,n

VO,2

+

-

+

-

+

-

Load

T1

T2

T3

T4

SUBMULTILEVEL CONVERTER Polarity Creator

Figure 2: The Proposed Topology for Symmetric and Asymmetric Inverter

A Novel Hybrid Asymmetric Cascaded Multilevel PWM Inverter for Reduction of THD 13


Figure 3: (a) Output of Submultilevel Converter (b) Typical Output Waveform of Nine Level Multilevel Inverter

Figure 4: Seven Level Asymmetric Inverter with Proposed Topology

Fundamental Frequency Modulation

In this technique gate signals of inverter are derived in such a way to get all the levels in the output waveform.

Here H-bridge is operated to generate polarities of ac output. For seven level inverter as shown in Figure 4 control signals

are given in the Figure 5. Ø1, Ø2, and Ø3 are calculated such that major harmonics in the output voltage waveform are

eliminated [8]. These values are called as optimum angles and these can be calculated by using control algorithms.

Advantage of this technique is that it uses low switching frequency over the PWM control techniques. The main

disadvantage is the presence of lower order harmonics in the output because of which the filtering cost increases.





Figure 5: Seven Level Inverter with Fundamental Frequency Operation

Pulse Width Modulation

For the given proposed asymmetric multilevel inverter special PWM techniques are required in order to get the

controlled output. Since the same configuration is used for symmetrical and asymmetrical inverter, same PWM technique

can be applied to the both the inverters. But here it is difficult to apply the gate signals directly to the asymmetric inverter

since operation of this inverter needs the random distribution of turn on of the switches in each half cycle of output.

The proposed PWM technique shown in Figure 6 gives the contiguous gate signals which are not suitable for the specific

arrangement of turn on of the inverters in this asymmetric topology. By using logic circuits PWM control signal can be

modified in such a way that it is suitable for the specific arrangement of turn on of the switches in the inverter.

For example for seven level asymmetric inverter PWM operation control signals are derived from the same level

symmetric inverter PWM control signals.

Control signals for seven level asymmetrical inverter are fabricated by using following mathematical

equations (1-3).

(1)

(2)

(3)

Where,

Table 1

f(t) reference signal,

N number of level, n=7,

ma modulation index (0-1.0),

r(t) PWM reference signal,

C1(t) Multiplexing signal.

Three carrier signals are required above the zero reference and in general (n-1)/2 carrier signals are required

where n is the number of levels. Multiplexing signals for obtaining given gate pattern varies with the number of levels.

Here C1 is multiplexing signal and it is used to obtain the PWM gate signal for switch S1 with the help of logic circuit

shown in Figure 7. From the output of seven level inverter it is observed that fundamental component of output is more



deviated from the approximate sine wave. This results in high value of THD and poor voltage quality. This is mainly due to

the unequal pulses at each step of multilevel output when high voltage source is operated alone to obtain higher levels of

the multilevel output. For example seven level hybrid asymmetric inverter output given in Figure 8 shows that when higher

voltage source operated alone in PWM mode it results 2V1 magnitude pulses in the second step of output and in the

remaining two steps the pulse magnitude is V1 since the same source gives the pulsed output in these steps. Thus this type

of PWM operation results in more THD and poor output quality as compared to fundamental frequency modulation.

Figure 6: Control Signals for Proposed Seven Level Asymmetrical Inverter

PWM1

PWM2

C1

PWM3

Vdc

T1

T3

T2

T4

2Vdc

Main inv

load

S3

S4

S1

S2

Figure 7: Logic Circuit for Obtaining Gate Signals

Figure 8: Output of Seven Level Asymmetrical Inverter with Proposed PWM Operation





Even though if the number of levels are increased inherently with asymmetric topology but with this type of

PWM operation, output gets poor quality even the levels are increasing. Seven level asymmetric inverter results in a

THD=28.97 %which is far away from the value 17.17% with fundamental frequency modulation and 17.93 % of its

counterpart symmetric topology. Apart from this when high voltage source acts alone in the output, it causes high

dv/dt across the switch which adversely affects the high frequency switches.

The main advantage of asymmetric topology is that as the number of levels are increased per phase with the given

number of sources as compared to symmetric topology, it results in better output quality. But during PWM operation of the

asymmetric inverter, it loses the above advantage of the better quality output. To maintain its advantages during PWM

mode operation a modified PWM technique is proposed.

PROPOSED HYBRID ASYMMETRICAL INVERTER

In the proposed hybrid inverter, the basic blocks have same rated source as that of the lowest rating source of

asymmetric inverter. These are added in the sub multilevel inverter of asymmetrical inverter as shown in Figure 9.

The basic blocks are grouped and named as symmetric part and the remaining blocks are called as asymmetric part.

To eliminate the unequal pulses in the output of asymmetrical inverter, high voltage sources in the inverter should

not give pulsed output when they are operated alone. Instead of operating an asymmetric part in the PWM mode,

symmetric part of the hybrid inverter is operated in PWM mode. Since symmetric part of the inverter uses same rating

sources, the pulses in the multilevel output are equal. Asymmetric part of the inverter is operated in square wave mode and

it gives square wave type output. Finally asymmetric part is used for level creation and symmetric part is used for equal

pulses in each step. Thus the multilevel output is better controlled and gives less THD which is equal to the same level of

symmetric topology. By adding the symmetric part to the asymmetric inverter, the number of levels increased in the output

of given asymmetric inverter. Number of output voltage levels possible with the proposed hybrid asymmetrical inverter are

given by equation (4) and number of IGBTs are given in the equation (5).

V1

V1

V2

Submultilevel

inverter

Asymmetric part

Symmetric part

V1<V2

H-bridge

Polarity creator

V0

Basic block

Figure 9: Proposed Hybrid Asymmetrical Inverter



Nlevel=(2(n+1)

– 1) + 2m (4)

NIGBT= 2(n+m) + 4 (5)

Where n is the number of basic blocks of asymmetric part and m is the number of basic blocks of symmetric part.

Possible levels with this type of inverter are 3, 5, 7, 9, 11, 13, 15, 17, 19, and …. for different values of n and m

combinations where n, m are positive integers. So with this hybrid configuration all odd levels are possible which was not

possible with the proposed asymmetric inverter. Number of basic blocks in the symmetric part depends upon the required

level output which is not possible with asymmetric inverter alone. For example nine and eleven level output is not possible

with the proposed asymmetric inverter to get these levels with hybrid asymmetric inverter n=2 is used in both the inverters

but m=1 and m=2 are the number of basic blocks used in symmetric part of the two inverters respectively.

Since symmetric part is operating at high frequency, the dv/dt during switching is low as compared to that of the

asymmetric inverter [9]. Nine level inverter is formed with one basic block in symmetric part and two blocks in

asymmetric part as shown in Figure 10. In asymmetric part the values of voltage sources are V1=50 V and V2=100 V. Since

V1 is the lowest rating of voltage source, same value of voltage source should be connected in symmetric part.

Figure 10: Nine Level Hybrid Asymmetrical Inverter

Figure 11: Logic Circuit for Symmetric Part





Figure 12: Control Signals of Level Shifted PWM Technique and Multiplex Signals

Logic circuits are used for the generation of gate pulses for symmetric part and asymmetric part. Symmetric part

logic circuit is shown in Figure 11and by using multiplex signals shown in Figure 12. Asymmetrical part gate signals are

similar to gate signals of seven level inverter shown in Figure 5. Simulation output of nine level hybrid symmetrical

inverter is shown in Figure 13.

Figure 13: Output of Nine Level Inverter



Figure 14: Simulation Output of Twenty Three Level Hybrid Asymmetrical Inverter

Figure 15: Simulation Output of Twenty Five Level Hybrid Asymmetrical Inverter





Twenty three level inverter can be formed by using five cells in symmetric part and three cells in asymmetric part

i.e n=3, m=4. In simulation model 50 V sources are used in symmetric part and 50 V, 100 V, and 200 V sources in

asymmetric part. Simulation output is shown in Figure 14.

Twenty five level inverter can be formed by using five cells in symmetric part and three cells in asymmetric part

i.e n=3, m=5. In simulation model 50 V sources are used in symmetric part and 50 V, 100 V, and 200 V sources in

asymmetric part. Simulation output is shown in Figure 15. The proposed hybrid asymmetrical and symmetrical inverter

gives same THD when they are operated in PWM mode for the same level output. For example ninelevel symmetrical and

hybrid asymmetrical inverter gives the same THD i.e 13.78%.

CONCLUSIONS

In this paper a new multilevel inverter topology is proposed by using series-connected sub multilevel inverters.

The proposed multilevel inverter uses reduced number of switches. Same topology can be used as symmetric and

asymmetric inverter. Modified PWM techniques are proposed for the better control of asymmetric topology and these are

derived from level shifted PWM technique. Without increasing basic blocks in the symmetric topology for more levels in

the output asymmetric topology is proposed with different dc sources.

Fundamental frequency control is easy to apply in asymmetric topology. But PWM operation gives more

THD and poor output quality. When asymmetrical inverter operated in PWM mode to improve output quality hybrid

asymmetrical inverter is proposed this gives the same value of THD as symmetrical topology. Matlab/Simulink models are

used for the study of hybrid asymmetric inverter. Since the proposed asymmetrical topology has more advantages over

other topologies these can be used in Wind-PV hybrid generating systems.

REFERENCES

1. Mohammad FarhadiKangarlu and EbrahimBabaei, “A generalized cascaded multilevel inverter using series

connection of submultilevel inverters”, IEEE Trans. on Power Electronics, vol. 28, no. 2, Feb. 2013.

2. Alex Ruderman and Sam Schlosberg, “A hybrid asymmetric cascaded multilevel inverter comprising high

resolution and symmetric low resolution parts”, IEEE 25th Convention of Electrical and Electronics Engineers in

Israel (IEEEI- 2008), pp. 21-25, 3-5 Dec. 2008.

3. EbrahimBabaei and Mohammad SadeghMoeinian, “Asymmetric cascaded multilevel inverter with charge balance

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4. Khomfoi and L. M. Tolbert, Multilevel power converters Power Electronics Handbook, 2nd Edition Elsevier,

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5. EbrahimBabaei and Seyed Hossein Hosseini, “New multilevel converter topology with minimum number of gate

driver circuits”, International Symposium on Power Electronics, Electrical Drives, Automation and Motion

(SPEEDAM), pp. 792-797, 2008.



6. EbrahimBabaei, Mohammad FarhadiKangarlu, and FarshidNajatyMazgar, “Symmetric and asymmetric multilevel

inverter topologies with reduced switching devices”, Elsevier Journal of Electric Power Systems Research,

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7. EbrahimBabaei and SeyedHosseinHosseini, “New cascaded multilevel inverter topology with minimum number

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